Many variant alleles of Brca1 and Brca2 are highly penetrant and predispose carriers to early onset breast cancer. These alleles, however, account for a minority of familial breast cancer cases. It is likely that the majority of such cases are mediated by low penetrance alleles in other genes involved in DNA damage response or DNA repair. Among these is the gene that encodes the cell cycle checkpoint kinase 2 (CHEK2), which is the focus of this application. In particular, we will focus on a CHEK2 (Chk2 in mouse) allele, designated CHEK2*1100delC, for which we have made a knockin mouse model. The CHEK2*1100delC allele, which predisposes to breast cancer in humans, has a single nucleotide deletion near its 3' end and encodes a C-terminal truncated protein lacking much of the kinase domain. Significantly, this allele confers increased breast cancer risk, but not in patients who have variant BRCA1 or BRCA2 alleles, suggesting that CHEK2 participates in the same pathway as the BRCA proteins. The CHEK2 kinase is a participant in a cellular DNA damage response, and interacts with other members of the pathway, including BRCA1 and BRCA2. CHEK2 phosphorylates BRCA1, and we now provide evidence that CHEK2 also phosphorylates BRCA2, a protein involved in recombination-mediated DNA repair. We demonstrate that CHEK2 phosphorylates BRCA2 at residues important for the association of BRCA2 with Rad51 and for the deposition of Rad51 onto single strand DNA, an activity required for efficient recombination-mediated DNA repair. We propose to utilize our Chk2*1100delC mouse knockin model to test the hypothesis that Chk2 kinase activity regulates the function of BRCA2 in recombination-mediated DNA repair, at least in part. Further, we will test whether the absence or reduction in Chk2 kinase activity, and resultant genomic instability, contributes to increased risk of cancer, particularly as a consequence of environmental exposure. [unreadable] [unreadable] [unreadable] [unreadable]